Partial amino acid sequences have been determined for the Ca2+/calmodulin-stimulated cyclic nucleotide phosphodiesterase from bovine brain and the cGMP-stimulated cyclic nucleotide phosphodiesterase from bovine heart. Examination of these sequences for homologous segments and comparison with protein sequences derived from the nucleotide sequences of the yeast PDE2 gene and the Drosophila dunce+ gene [Chen, C.-N., Denome, S. & Davis, R. L. (1986) Proc. Natl. Acad. Sci. USA 83, 9313-9317; Sass, P., Field, J., Nikawa, J., Toda, T. & Wigler, M. (1986) Proc. Natl. Acad. Sci. USA 83, 9303-9307] reveal a 200- to 270-residue segment in each that is homologous to the others. The molecular masses of the four proteins vary from 40 kDa to 105 kDa, and the structural resemblance appears to be constrained to a single segment of each protein. These related segments are proposed to comprise the catalytic domains in this set of enzymes. The lack of absolute sequence identity between the two bovine enzymes shows that they are unique gene products that are not produced by alternative processing of a larger protein or of a single mRNA precursor. The data also strongly support the conclusion that the dunce+ gene locus of Drosophila and the PDE2 gene locus in yeast code for structural genes of cyclic nucleotide phosphodiesterases.